This invention relates to eyepiece variable focal length optics for use in combination with objective optics as in astronomical telescopes, spotting scopes and binoculars.
Many proposals have so far been made in the art of eyepiece variable focal length optics of the type contemplated by the invention. For example, U.S. Pat. No. 5,491,588 proposes a focal variable focal length optics which is a combination of an eyepiece variable focal length optical system with an objective optical system and an erecting optical system. Unexamined Published Japanese Patent Application (kokai) No. 134617/1987 and U.S. Pat. No. 5,734,509 propose independent eyepiece variable focal length optics which do not include an objective optical system or an erecting optical system. The eyepiece variable focal length system, taken alone and excluding the objective and erecting optical systems, is a so-called OE-ZOOM system consisting of the following three groups in order from the object side, a moving first lens group having negative power, a moving second lens group arranged to be opposed to the first lens group with a field stop interposed therebetween and having positive power, and a fixed third lens group having positive power. The first and second lens groups are moved in opposite directions not only to achieve variable focal length but also to compensate for the position of the virtual image. The conventional eyepiece variable focal length optics mentioned above can achieve variable power ratios of only about two.
Eyepiece optics are generally required to have adequate eye relief and offer a satisfactorily large apparent visual field; however, if this need is met, the optics are prone to become bulky. This tendency becomes noticeable as the variable power ratio approaches three.
An object, therefore, of the invention is to provide eyepiece variable focal length optics that achieves a variable power ratio of about three and which yet is compact and allows for adequately large apparent visual field and long eye relief.
This object of the invention can be attained by eyepiece variable focal length optics which comprises, in order from objective optics with which it is used in combination, a negative first lens group, a positive second lens group and a positive third lens group, the first and second lens groups moving in opposite directions along the optical axis during variable focal length but the third lens group being stationary, and the eyepiece variable focal length optics satisfying the following conditions (1) and (2):
2.0 less than f3/f2 less than 4.0xe2x80x83xe2x80x83(1)
7.0 less than L12/f(h)xe2x80x83xe2x80x83(2)
where f2 is the focal length of the second lens group, f3 is the focal length of the third lens group, f(h) is the shortest focal length of the overall eyepiece variable focal length optics, and L12 is the axial air gap between the first and second lens groups at the shortest focal length.
When the overall eyepiece variable focal length optics is set at the shortest focal length state, a telescope using the eyepiece variable focal length optics in combination with the objective optics provides the highest magnification, assuming that the objective optics has a fixed focal length. Of course, the objective optics may be constructed as a variable focal length objective optics to further increase the highest magnification.
Preferably, the second lens group contains at least two positive lens elements and satisfies the following condition (3):
0.3 less than f2/f2xe2x88x921 less than 0.6xe2x80x83xe2x80x83(3)
where f2xe2x88x921 is the focal length of the positive lens in the second lens group which is positioned the closest to the objective optics (if said positive lens is part of a cemented lens, f2xe2x88x921 is the focal length of the cemented lens).
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2000-53732 (filed on Feb. 29, 2000), which is expressly incorporated herein by reference in its entirety.